The gas likely burst forth as little as one million to ten million years ago.

That would make the features relatively young, since any time less than ten million years ago is "like yesterday" in comparison to the moon's age, said Peter H. Schultz, a planetary geologist at Brown University in Providence, Rhode Island, and lead study author.

The finding could prove that the moon is still an active body, countering the long-standing belief that there has been no major lunar volcanism in the past three billion years.

Reporting in tomorrow's issue of the journal Nature, Schultz and colleagues suggest that such outgassing could be responsible for the formation of four to ten similar sites on the moon—and could still be occurring today.

Colorful Revelation

Scientists have suspected that the moon's D-shaped Ina structure is unique ever since Apollo 15 astronauts took images of the feature while flying overhead in 1971.